Thin-film solar cells of the above-described crystal structure have possibilities for extensive practical use, as described in U.S. Pat. No. 4,335,226 (issued to Michelsen et al. on Jun 15, 1982). Disclosed in the U.S. patent document cited above are a method for producing a light absorbing layer consisting of a single phase of CuInSe.sub.2 (CIS) having a homogeneous structure and a method for producing a thin-film solar cell employing the light absorbing layer and having a high conversion efficiency.
Such thin-film solar cells having a high conversion efficiency are disclosed in, e.g., U.S. Pat. Nos. 4,798,660 (issued to J. H. Ermer et al.) and 4,915,745 (issued to G. A. Pollock et al.); C. L. Jensen et al., Proceedings 23rd Photovoltaic Specialists Conference, (1993), p. 577; and JP-A-4-326526 (Mitsune et al.). (The term "JP-A" as used herein means an "unexamined published Japanese patent application.")
U.S. Pat. No. 4,798,660 discloses a method for forming a thin-film light absorbing layer consisting of a CIS single phase having a uniform composition, which comprises depositing a metallic back electrode layer, a layer of pure copper alone, and a layer of pure indium alone in this order by DC magnetron sputtering to form a stacked thin metallic film and then reacting the stacked metallic film with selenium by exposing the same to a selenium atmosphere, desirably an H.sub.2 Se gas atmosphere.
U.S. Pat. No. 4,915,745 discloses a technique for improving the performance of a thin-film solar cell by forming a stacked precursor film comprising a copper-gallium alloy layer containing gallium and a layer of pure indium and then heating the precursor film in a selenium atmosphere to form a light absorbing layer, whereby gallium segregates and migrates to the metallic back electrode layer of molybdenum (Mo) during the heat treatment to function as a kind of adhesive to improve adhesion between the metallic back electrode layer and the resulting thin-film light absorbing layer.
C. L. Jensen et al. suggested in the reference cited above that the CIS light absorbing layer formed by heating a stacked precursor film comprising a copper-gallium alloy layer containing gallium and a layer of pure indium in a selenium atmosphere has improved adhesion to the metallic back electrode layer of molybdenum (Mo) because of the segregation of gallium and migration thereof to the metallic back electrode layer during the heat treatment, and that there is the possibility that the light absorbing layer formed has an internal structure having a gallium concentration gradient and made up of two layers, i.e., a Cu(InGa)Se.sub.2 layer and a CIS layer, based on the results of Auger electron spectroscopy (AES).
JP-A-4-326526 discloses a method for producing a thin film consisting only of a single phase of copper indium gallium diselenide sulfide which comprises forming a molybdenum layer on a Corning 7059 (trade name of Corning Inc.) glass substrate by sputtering, subsequently forming thereon a stacked film consisting of copper, indium, and sulfur by vapor deposition, heating the stacked film in a nitrogen atmosphere to form a thin film consisting of a single phase of copper indium gallium disulfide, and then introducing H.sub.2 Se gas into the same oven to convert the thin film to the desired one. There is a description in the above reference to the effect that a thin film consisting of a single phase of copper indium gallium diselenide sulfide having the same performance can also be obtained by a method which is the same as the above, except that selenium is used in place of sulfur and that H.sub.2 S gas is used in place of H.sub.2 Se gas as the atmosphere for heat treatment.
For producing a thin-film light absorbing layer made of a Cu-III-VI.sub.2 chalcopyrite semiconductor, e.g., a p-type semiconductor such as copper indium diselenide (CIS) or copper indium gallium diselenide (CIGS), coevaporation technique may be used which employs solid starting materials for these constituent elements. Since this coevaporation technique has an advantage that the content of gallium in the thin-film light absorbing layer can be freely regulated in the range of from 0 to 100 at. %, the optimum bandgap of 1.3 to 1.5 eV is attainable which is desirable to obtain better match with the air mass (AM) 1.5 solar spectrum changing the fractional Ga content in Cu(InGa)Se.sub.2 (CIGS) thin films. However, it has problems that production of a large-area thin-film solar cell is not easy because of the structure of the production equipment, and that the technique is unsuitable for mass production.
Furthermore, when a CIGS single phase is formed in a high gallium content, it is preferred to use a substrate having temperature not lower than the softening point of the soda-lime float glass, which is an inexpensive substrate generally employed, so that treatment at a higher temperature can be conducted for accelerating the diffusion of gallium. However, use of such a high substrate temperature poses problems of, for example, deformation of the soda-lime float glass, generation of unfavorable phases caused by the diffusion of an alkali ingredient from the glass into the CIGS phase, impaired reproducibility because of difficulties in diminishing the diffusion, and peeling of the thin-film light absorbing layer consisting of a CIGS single phase from the Mo back-electrode layer.